Reaction mechanism of the reduction of nitrogen monoxide by methane in an oxygen excess atmosphere (NO-CH 4 -O 2 reaction) catalyzed by Pd/H-ZSM-5 has been studied at 623-703K in the absence of water vapor, in comparison with the mechanism for Co-ZSM-5. Kinetic isotope effect for the N 2 formation in NO-CH 4 -O 2 vs. NO-CD 4 -O 2 reactions was 1.65 at 673K and decreased with a decrease in the reaction temperature. In addition, H-D isotopic exchange took place significantly in NO-(CH 4 +CD 4 )-O 2 reaction. These results are in marked contrast with the case of Co-ZSM-5, for which the C-H dissociation of methane is the only rate-determining step, and show that the C-H dissociation is slow but not the only rate-determining step in the case of Pd/H-ZSM-5.A reaction scheme was proposed, in which the relative rates of the three steps ((i)-(iii) below) vary depending on the reaction conditions.Further, in contrast to Co-ZSM-5, NO x -CH 4 -O 2 reaction was much slower than CH 4 -O 2 reaction for Pd/H-ZSM-5; the presence of NO x retards the reaction of CH 4 over the latter catalyst, while it accelerates the reaction over the former. It is suggested that CH 4 is activated directly by the Pd atoms in the case of Pd/H-ZSM-5, but by NO 2 strongly adsorbed on Co ion for Co-ZSM-5. The reaction order of the NO-CH 4 -O 2 reaction with respect to NO pressure was consistent with this mechanism; 1.05 for Pd/H-ZSM-5 and 0.11 for Co-ZSM-5.